Ductile iron

ABSTRACT

A ductile iron having superior high-temperature strength and oxidation resistance at temperatures exceeding 800° C. compared with conventional high Si and Mo ductile iron. The ductile iron comprises, in terms of mass ratio, carbon: 2.0 to 4.0%, silicon: 3.5 to 5.0%, manganese: not more than 1.0%, chromium: 0.1 to 1.0%, molybdenum: 0.2 to 2.0%, vanadium: 0.1 to 1.0%, and magnesium: 0.02 to 0.1%, with the remainder being composed of iron and unavoidable impurities.

TECHNICAL FIELD

The present invention provides a ductile iron having superiorhigh-temperature strength and oxidation resistance.

BACKGROUND ART

Ductile iron exhibits excellent high-temperature strength and oxidationresistance, and is used in turbine housings and exhaust manifolds ofturbocharger in the diesel engines of passenger vehicles and industrialmachinery, and the like. In recent years, improvements in fuelconsumption driven by environmental regulations have resulted in atendency for increased engine exhaust gas temperatures. Turbine housingsand exhaust manifolds are used under conditions where they are subjectedto rapid temperature variation as a result of repeated exposure to hightemperatures generated by the exhaust gases, and therefore requiresuperior levels of high-temperature strength and oxidation resistance.

A high Si and Mo ductile iron (ductile cast iron) is conventionally usedas the material for turbine housings, and the service temperature limitis typically not more than 800° C. However, in recent years there havebeen growing demands for turbine housings that can be used attemperatures exceeding 800° C.

Examples of other turbine housing materials having superior levels ofhigh-temperature strength and oxidation resistance that may be usedinstead of high Si and Mo ductile iron include Ni-resist cast iron andstainless cast iron. However, these materials include large amounts ofNi and Cr within the raw materials, meaning the raw material costs arehigh.

Accordingly, investigations are being conducted into improving ductileiron by appropriate alloy design, thereby improving the high-temperatureproperties such as the heat resistance. For example, patent citation 1discloses a ductile iron prepared by adding V to a high Si and Mo castiron.

PRIOR ART CITATIONS Patent Citations

-   Patent Citation 1: Publication of Japanese Patent No. 3,936,849

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, although the ductile iron of patent citation 1 was capable ofimproving the high-temperature strength at temperatures exceeding 800°C., the oxidation resistance was unsatisfactory.

The present invention has an object of providing a ductile iron that hasimproved levels of high-temperature strength and oxidation resistancecompared with conventional high Si and Mo ductile iron as well assuperior ductibility.

Means to Solve the Problems

In order to achieve the above object, the ductile iron of the presentinvention comprises, in terms of mass ratio, carbon: 2.0 to 4.0%,silicon: 3.5 to 5.0%, manganese: not more than 1.0%, chromium: 0.1 to1.0%, molybdenum: 0.2 to 2.0%, vanadium: 0.1 to 1.0%, and magnesium:0.02 to 0.1%, with the remainder being composed of iron and unavoidableimpurities.

In the ductile iron of the present invention, the molybdenum content isoptimized, and therefore the cast iron has excellent high-temperaturestrength as well as superior ductibility. Furthermore, the ductile ironof the present invention also comprises chromium, and because thechromium content is optimized, the cast iron exhibits superior oxidationresistance and ductibility. As a result, the ductile iron of the presentinvention can be used even under temperature conditions of 800° C. orhigher. Furthermore, production can be conducted at lower cost than thatof Ni-resist cast iron or stainless cast steel.

Further, the ductile iron described above preferably further comprises amass ratio of tungsten: 0.1 to 1.0%. Alternatively, the ductile ironpreferably further comprises niobium: 0.02 to 0.30%. Alternatively, theductile iron preferably further comprises tungsten: 0.1 to 1.0% andniobium: 0.02 to 0.30%.

In this manner, by optimizing the amount of tungsten or niobium, or theamounts of both tungsten and niobium, the high-temperature strength canbe further improved.

Reasons for restricting the amount of each of the components aredescribed below.

Carbon (C): C and Si are extremely important elements in cast iron. Ifthe C content is 2.0 mass % or less, then carbides tend to form readily,whereas a C content of 4.0 mass % or greater tends to induce graphitesegregation (carbon dross), resulting in a deterioration in the strengthand ductibility. Accordingly, the C content is specified as 2.0 to 4.0mass %. Further, the carbon equivalent value (CE=C %+0.31Si %) is usedas an indicator of the castability of the cast iron. The CE value of atypical ductile iron is within a range from 4.3 to 4.5. If this CE valueis 4.3 or less, then defects tend to form more easily, whereas a CEvalue of 4.5 or greater tends to induce carbon dross. In the presentinvention, because the Si content is set to a high value as describedbelow, the C content is preferably within a range from 2.7 to 3.2 mass%.

Silicon (Si): Si has the effects of promoting the graphitization of Cand the ferritization of the matrix. The Si content in a typical ductileiron is approximately 2.5 mass %. In the present invention, the Sicontent is not less than 3.5 mass %. Further, because the toughness ofthe cast iron deteriorates as the Si content is increased, the upperlimit for the Si content is 5.0 mass %. In order to further enhance theoxidation resistance, Si is preferably added in an amount of 4.3% orgreater, but because the ductibility of the cast iron decreases andcastability decreases due to increase of the CE value as the Si contentis increased, the upper limit for the Si content is preferably 4.7 mass%.

Manganese (Mn): Mn is an element that is necessary for fixing the S thatexists as an unavoidable impurity within the raw material as MnS,thereby rendering the S harmless. However, because Mn also causesformation of matrix pearlite structures, the upper limit for the Mncontent is specified as 1.0 mass %.

Molybdenum (Mo): Mo is an element that undergoes solid dissolutionwithin the matrix, thereby improving the tensile strength and yieldstrength at high temperatures. In the present invention, Mo is added inan amount of not less than 0.2 mass %. Moreover, in order to furtherimprove the heat resistance, the addition of 0.4 mass % or more isparticularly desirable. If the Mo content is too high, Mo and C tend tobond together to form carbides, and this causes the hardness to increaseand the ductibility to deteriorate. Accordingly, the upper limit for theMo content is specified as 2.0 mass %. In order to ensure no loss incutting properties, the upper limit for the Mo content is preferably 1.0mass %.

Vanadium (V): V is an element that is precipitated as fine carbideswithin the matrix, causing an increase in the tensile strength and yieldstrength at high temperatures. In the present invention, V is added inan amount of not less than 0.1 mass %. If the V content is too high,then the ductibility of the cast iron deteriorates, and therefore theupper limit for the V content is specified as 1.0 mass %. Further,because V has a strong tendency to form carbides, it tends to impede thespheroidization of C. Accordingly, the upper limit for the V content ispreferably 0.4 mass %.

Chromium (Cr): Cr is an element that improves the oxidation resistanceat high temperatures. In the present invention, Cr is added in an amountof not less than 0.1 mass %. In order to further enhance the oxidationresistance, addition of 0.2 mass % or more of Cr is preferred. If the Crcontent is too high, then the ductibility of the cast iron deteriorates,and therefore the upper limit for the Cr content is specified as 1.0mass %. Furthermore, Cr has a strong tendency to form carbides, meaningit impedes the spheroidization of C and tends to cause the size of thecarbide grains within the matrix to coarsen, and therefore the upperlimit for the Cr content is preferably 0.4 mass %.

Magnesium (Mg): Mg is added in an amount of not less than 0.02 mass %for the purpose of spheroidizing the graphite. However if the Mg contentis too high, then carbides are generated and dross defects (theincorporation of oxides) tend to occur, and therefore the upper limitfor the Mg content is specified as 0.1 mass %.

Tungsten (W): W, in a similar manner to Mo, is an element that undergoessolid dissolution within the matrix, thereby improving the tensilestrength and yield strength at high temperatures. In the presentinvention, W is added in an amount of not less than 0.1 mass %.Moreover, in order to further improve the heat resistance, the additionof 0.2 mass % or more is particularly desirable. Because W also has astrong tendency to form carbides, meaning it tends to impede thespheroidization of C, the upper limit for the W content is specified as1.0 mass %, and is preferably 0.4 mass %.

Niobium (Nb): Nb is an element that is precipitated as fine carbideswithin the matrix, causing an increase in the tensile strength and yieldstrength at high temperatures. In the present invention, Nb is added inan amount of not less than 0.02 mass %. If the Nb content is too high,then the ductibility of the cast iron deteriorates, and Nb also has astrong tendency to form carbides, meaning it impedes the spheroidizationof C and tends to cause the size of the carbide grains within the matrixto coarsen, and therefore the upper limit for the Nb content isspecified as 0.30 mass %. A preferred range for the amount of Nb, whichrealizes a marked strength improvement effect, prevents any significantdeterioration in the ductibility and enables an increase in thespheroidization rate of C, is from 0.04 to 0.20 mass %, and a morepreferred range is from 0.05 to 0.10 mass %.

In the above ductile iron, the spheroidization rate of the graphite ispreferably 90% or higher. At a graphite spheroidization rate of 90%, thetensile strength and yield strength at high temperatures can beimproved.

A turbine housing, exhaust manifold, and turbine housing-integratedexhaust manifold produced using the above ductile iron exhibit excellenthigh-temperature strength and oxidation resistance, and can be usedunder temperature conditions of 800° C. or higher.

Effect of the Invention

According to the present invention, by adopting the compositiondescribed above, a ductile iron having superior high-temperaturestrength and oxidation resistance as well as excellent ductibility canbe produced at low cost.

A turbine housing, exhaust manifold, and turbine housing-integratedexhaust manifold produced using the ductile iron of the presentinvention are able to satisfactorily withstand usage underhigh-temperature conditions of 800° C. or higher.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A graph illustrating the 0.2% yield strength ratios for testpieces of sample numbers 1 to 13, referenced against the test piece ofsample number 1.

FIG. 2 A graph illustrating the oxidation weight loss ratios for testpieces of sample numbers 1 to 13, referenced against the test piece ofsample number 1.

FIG. 3 A graph illustrating the relationship between the Mo content andthe test piece elongation ratio after fracture (referenced against thetest piece of sample number 1).

FIG. 4 A graph illustrating the relationship between the Cr content andthe test piece elongation ratio after fracture (referenced against thetest piece of sample number 1).

FIG. 5 A graph illustrating the 0.2% yield strength ratios for testpieces of sample numbers 1 and 14 to 18, referenced against the testpiece of sample number 1.

FIG. 6 A graph illustrating the oxidation weight loss ratios for testpieces of sample numbers 1 and 14 to 18, referenced against the testpiece of sample number 1.

FIG. 7 A graph illustrating the relationship between the W content andthe test piece elongation ratio after fracture (referenced against thetest piece of sample number 1).

FIG. 8 A graph illustrating the 0.2% yield strength ratios for testpieces of sample numbers 1 and 19 to 22, referenced against the testpiece of sample number 1.

FIG. 9 A graph illustrating the oxidation weight loss ratios for testpieces of sample numbers 1 and 19 to 22, referenced against the testpiece of sample number 1.

FIG. 10 A graph illustrating the 0.2% yield strength ratios for testpieces of sample numbers 1 and 23 to 26, referenced against the testpiece of sample number 1.

FIG. 11 A graph illustrating the oxidation weight loss ratios for testpieces of sample numbers 1 and 23 to 26, referenced against the testpiece of sample number 1.

FIG. 12 A graph illustrating the tensile strength ratios for test piecesof sample numbers 1, 15, 20, 24 and 31 to 34 (referenced against thetest piece of sample number 1).

BEST MODE FOR CARRYING OUT THE INVENTION

The ductile iron of the present invention is described below in moredetail based on a series of examples.

Example 1

Table 1 shows the element composition of ductile iron test pieces ofsample numbers 1 to 13.

TABLE 1 Sam- ple Num- Composition (mass %) ber C Si Mn Mo V W Cr Mg 12.98 4.68 0.42 0.41 0.29 — 0.32 0.043 2 2.97 4.67 0.39 0.22 0.28 — 0.320.042 3 3.03 4.62 0.40 0.82 0.30 — 0.30 0.040 4 2.99 4.64 0.40 1.83 0.28— 0.28 0.042 5 2.92 4.65 0.39 0.10 0.30 — 0.29 0.042 6 3.07 4.68 0.382.20 0.30 — 0.31 0.040 7 2.97 4.62 0.38 0.39 0.29 — 0.10 0.039 8 3.014.67 0.40 0.40 0.31 — 0.82 0.042 9 2.99 4.66 0.41 0.40 0.30 — 0.05 0.04310 3.00 4.65 0.41 0.39 0.31 — 1.20 0.040 11 3.06 4.63 0.39 0.40 0.30 — —0.042 12 1.80 4.62 0.40 0.39 0.30 — 0.30 0.040 13 3.06 3.30 0.40 0.390.28 — 0.28 0.042

Raw materials were blended and melted to achieve the elementcompositions detailed in Table 1, and each composition was then cast inthe shape of a Y-block B-type test piece prescribed in JIS G 5502, thuscompleting preparation of test pieces for sample numbers 1 to 13.Subsequently, each test piece was subjected to a heat treatment at 915°C. for 3 hours to effect ferritization.

Using the method prescribed in JIS G 5502, the spheroidization rate wasmeasured for sample numbers 1 to 13. The spheroidization rate was atleast 90% for each of the test pieces of sample numbers 1 to 11 andsample number 13. The spheroidization rate for the test piece of samplenumber 12 was 50%.

Each of the test pieces of ductile iron from sample numbers 1 to 13 wasmeasured for 0.2% yield strength and oxidation resistance at 800° C.,and for elongation after fracture at room temperature.

The oxidation resistance was evaluated using the oxidation weight loss.

The test piece was placed inside an electric furnace, and thetemperature was held at 800° C. for 100 hours under normal atmosphericconditions. Subsequently, the test piece was boiled in an aqueoussolution containing 18% NaOH and 3% KMnO₄, and then boiled in a 10%ammonium citrate solution, thereby removing any oxides from the surfaceof the test piece. The mass of the test piece was measured prior toheating and was then re-measured following removal of the oxides, andthe oxidation weight loss was calculated using formula (1).

W _(d)=(W ₀ −W _(s))/A ₀  (1)

wherein W_(d) represents the oxidation weight loss (mg/cm²), W_(s)represents the mass (mg) following testing, W₀ represents the mass (mg)prior to testing, and A₀ represents the surface area (cm²) of the testpiece prior to testing.

FIG. 1 illustrates the 0.2% yield strength ratio for each test piece,referenced against the ductile iron test piece of sample number 1. Inthis figure, the vertical axis represents the 0.2% yield strength ratio.FIG. 2 illustrates the oxidation weight loss ratio for each test piece,referenced against the ductile iron test piece of sample number 1. Inthis figure, the vertical axis represents the oxidation weight lossratio. FIG. 3 illustrates the relationship between the Mo content andthe elongation ratio after fracture of the test piece (referencedagainst the test piece of sample number 1). In this figure, thehorizontal axis represents the Mo content, and the vertical axisrepresents the elongation ratio after fracture. FIG. 4 illustrates therelationship between the Cr content and the elongation after fracture ofthe test piece (referenced against the test piece of sample number 1).In this figure, the horizontal axis represents the Cr content, and thevertical axis represents the elongation ratio after fracture.

In the test pieces of sample numbers 1 to 6, which had varying Mocontent values, it is evident from FIG. 1 that the 0.2% yield strengthincreased as the Mo content was increased. Further, as illustrated inFIG. 2, the oxidation weight loss was substantially uniform, and wasunaffected by the Mo content.

However, as illustrated in FIG. 3, the elongation after fracture(ductibility) deteriorated as the Mo content was increased.

In other words, by employing a Mo content within a range from 0.2 to 2.0mass %, a ductile iron having a combination of superior high-temperaturestrength and superior ductibility was able to be obtained.

In the test pieces of sample numbers 1 and 7 to 11, which had varying Crcontent values, it is evident from FIG. 2 that incorporating Cr reducedthe oxidation weight loss, and that increasing the Cr content enabledthe oxidation weight loss to be further reduced (namely, an improvementin the oxidation resistance). However, as illustrated in FIG. 4, theelongation after fracture (ductibility) deteriorated as the Cr contentwas increased.

Accordingly, by employing a Cr content within a range from 0.1 to 1 mass%, a ductile iron having a combination of superior oxidation resistanceand superior ductibility was able to be obtained.

In the test piece of sample number 12, which had a very low C content,carbides formed and spheroidization of the carbon was inhibited,resulting in a dramatic fall in the 0.2% yield strength. The test pieceof sample number 13, which had a low Si content, exhibited inferioroxidation resistance.

Example 2

Table 2 shows the element composition of ductile iron test pieces ofsample numbers 1 and 14 to 18.

TABLE 2 Sam- ple Num- Composition (mass %) ber C Si Mn Mo V W Cr Mg 12.98 4.68 0.42 0.41 0.29 — 0.32 0.043 14 2.99 4.62 0.40 0.40 0.29 0.180.30 0.039 15 3.01 4.68 0.40 0.41 0.29 0.31 0.33 0.043 16 3.03 4.60 0.410.40 0.30 0.95 0.32 0.045 17 2.98 4.64 0.42 0.39 0.29 0.07 0.30 0.043 183.01 4.68 0.40 0.41 0.29 1.21 0.31 0.044

Using the same method as example 1, test pieces were prepared usingsample numbers 14 to 18 and subsequently subjected to ferritization.Measurement of the spheroidization rate using the method described inJIS G 5502 revealed a spheroidization rate of at least 90% for each ofthe test pieces.

Each of the test pieces from sample numbers 14 to 18 was measured for0.2% yield strength and oxidation weight loss at 800° C. FIG. 5illustrates the 0.2% yield strength ratio for each test piece,referenced against the ductile iron test piece of sample number 1. Inthis figure, the vertical axis represents the 0.2% yield strength ratio.FIG. 6 illustrates the oxidation weight loss ratio for each test piece,referenced against the ductile iron test piece of sample number 1. Inthis figure, the vertical axis represents the oxidation weight lossratio. FIG. 7 illustrates the relationship between the W content and theelongation ratio after fracture of the test piece (referenced againstthe test piece of sample number 1). In this figure, the horizontal axisrepresents the W content, and the vertical axis represents theelongation ratio after fracture.

From the results for sample numbers 1, 14, 15 and 17 it was confirmedthat as the W content was increased, the W underwent solid dissolutionwithin the ferrite matrix, thereby strengthening the matrix andimproving the 0.2% yield strength. However, the results from samplematerials 16 and 18 revealed that addition of a very large amount of Wdid not result in a dramatic improvement in the high-temperaturestrength. As illustrated in FIG. 6, the oxidation weight loss wasindependent of the W content, with all of the test pieces exhibiting ahigh level of oxidation resistance. Further, as illustrated in FIG. 7,the elongation after fracture (ductibility) deteriorated as the Wcontent was increased.

Based on the above results it is clear that by employing a W contentwithin a range from 0.1 to 1 mass %, the high-temperature strength wasable to be further improved.

Example 3

Table 3 shows the element composition of ductile iron test pieces ofsample numbers 1 and 19 to 22.

TABLE 3 Sam- ple Num- Composition (mass %) ber C Si Mn Mo V Nb Cr Mg 12.98 4.68 0.42 0.41 0.29 — 0.32 0.043 19 3.03 4.58 0.41 0.41 0.31 0.040.32 0.041 20 3.00 4.63 0.40 0.41 0.29 0.09 0.31 0.040 21 3.03 4.61 0.400.39 0.30 0.27 0.32 0.045 22 3.04 4.60 0.43 0.39 0.30 0.35 0.31 0.041

Using the same method as example 1, test pieces were prepared with theelement compositions detailed for sample numbers 19 to 22. Following ahomogenized heat treatment for one hour at 1,200° C., a heat treatmentwas performed at 915° C. for 3 hours to effect ferritization.Measurement of the spheroidization rate using the method described inJIS G 5502 confirmed a spheroidization rate of at least 90% for each ofthe test pieces. Subsequently, each of the test pieces was measured for0.2% yield strength and oxidation weight loss at 800° C.

FIG. 8 illustrates the 0.2% yield strength ratio for each test piece,referenced against the ductile iron test piece of sample number 1. Inthis figure, the vertical axis represents the 0.2% yield strength ratio.FIG. 9 illustrates the oxidation weight loss ratio for each test piece,referenced against the ductile iron test piece of sample number 1. Inthis figure, the vertical axis represents the oxidation weight lossratio.

From the results for sample numbers 19 and 20 it was confirmed thatsince Nb is an element that is precipitated as fine carbides within thematrix, the matrix is strengthened and the 0.2% yield strength isimproved as the Nb content was increased. However, the results fromsample materials 21 and 22 revealed that an additional increase in theamount of Nb actually tended to cause a reduction in the 0.2% yieldstrength. In particular, sample number 22 exhibited a lower 0.2% yieldstrength than that of sample number 1 which contained no Nb. Moreover,as illustrated in FIG. 9, the oxidation weight loss was independent ofthe Nb content and remained substantially constant.

In other words, by employing a Nb content within a range from 0.02 to0.3 mass %, the high-temperature strength was able to be furtherimproved.

Example 4

Table 4 shows the element composition of ductile iron test pieces ofsample numbers 1 and 23 to 26.

TABLE 4 Sam- ple Num- Composition (mass %) ber C Si Mn Mo V W Nb Cr Mg 12.98 4.68 0.42 0.41 0.29 — — 0.32 0.043 23 3.02 4.64 0.40 0.42 0.30 0.300.04 0.34 0.043 24 3.01 4.67 0.39 0.41 0.29 0.32 0.08 0.35 0.041 25 2.984.68 0.41 0.40 0.29 0.31 0.26 0.33 0.044 26 3.01 4.68 0.41 0.41 0.310.31 0.35 0.34 0.040

Using the same method as example 1, test pieces were prepared with theelement compositions detailed for sample numbers 23 to 26. Subsequently,a homogenized heat treatment was performed in the same manner as example3, followed by ferritization. Measurement of the spheroidization rateusing the method described in JIS G 5502 confirmed a spheroidizationrate of at least 90% for each of the test pieces. Subsequently, each ofthe test pieces was measured for 0.2% yield strength and oxidationweight loss at 800° C.

FIG. 10 illustrates the 0.2% yield strength ratio for each test piece,referenced against the ductile iron test piece of sample number 1. Inthis figure, the vertical axis represents the 0.2% yield strength ratio.FIG. 11 illustrates the oxidation weight loss ratio for each test piece,referenced against the ductile iron test piece of sample number 1. Inthis figure, the vertical axis represents the oxidation weight lossratio.

The results for sample numbers 1, 23 and 24 illustrate that increasingthe Nb content improved the 0.2% yield strength. In particular, samplenumber 24 exhibited a higher 0.2% yield strength than samples containingonly one of Nb and W. In the test pieces of sample numbers 25 and 26,which had an even higher Nb content, the 0.2% yield strength actuallydecreased. The test piece of sample number 26 exhibited a lower 0.2%yield strength than the test piece of sample number 1 which contained noadded Nb or W. Moreover, as illustrated in FIG. 11, the oxidation weightloss was independent of the Nb content and remained substantiallyconstant.

In other words, by including both W and Nb, the high-temperaturestrength was able to be further improved.

Example 5

The Mg content was reduced for the element compositions of samplenumbers 1, 15, 20 and 24, and test pieces were prepared from theresulting sample numbers 31 to 34. The element composition of each ofthe test pieces is shown in Table 5.

TABLE 5 Sample Composition (mass %) Spheroidization Number C Si Mn Mo VW Nb Cr Mg rate (%) 1 2.98 4.68 0.42 0.41 0.29 — — 0.32 0.043 >90 153.01 4.68 0.40 0.41 0.29 0.31 — 0.33 0.043 >90 20 3.00 4.63 0.40 0.410.29 — 0.09 0.31 0.040 >90 24 3.01 4.67 0.39 0.41 0.29 0.32 0.08 0.350.041 >90 31 3.03 4.60 0.40 0.39 0.30 — — 0.29 0.020 51 32 2.98 4.640.40 0.39 0.31 0.29 — 0.30 0.023 45 33 3.02 4.69 0.40 0.39 0.31 — 0.090.31 0.023 43 34 2.95 4.61 0.39 0.41 0.30 0.30 0.08 0.31 0.021 40

Using the same method as example 3, test pieces were prepared with eachof the element compositions detailed in Table 5, and a homogenized heattreatment was then performed, followed by ferritization. Thespheroidization rate of each test piece was measured using the methoddescribed in JIS G 5502. The tensile strength of each test piece at 800°C. was also measured.

FIG. 12 illustrates the tensile strength ratio for each test piece,referenced against the test piece of sample number 1. In this figure,the vertical axis represents the tensile strength ratio. As the Mgcontent was reduced, the spheroidization rate decreased. Accompanyingthis decrease, the tensile strength at 800° C. also decreased.

In this manner, by ensuring that the spheroidization rate was at least90%, the high-temperature strength was able to be increased.

1. A ductile iron comprising, in terms of mass ratio, carbon: 2.0 to4.0%, silicon: 3.5 to 5.0%, manganese: not more than 1.0%, chromium: 0.1to 1.0%, molybdenum: 0.2 to 2.0%, vanadium: 0.1 to 1.0%, and magnesium:0.02 to 0.1%, with a remainder being composed of iron and unavoidableimpurities.
 2. The ductile iron according to claim 1, further comprisinga mass ratio of tungsten: 0.1 to 1.0%.
 3. The ductile iron according toclaim 1, further comprising a mass ratio of niobium: 0.02 to 0.30%. 4.The ductile iron according to claim 1, further comprising, in terms ofmass ratio, tungsten: 0.1 to 1.0% and niobium: 0.02 to 0.30%.
 5. Theductile iron according to claim 1, wherein the spheroidization rate ofgraphite is 90% or higher.
 6. An exhaust system component, producedusing the ductile iron according to claim
 1. 7. The exhaust systemcomponent according to claim 6, wherein the exhaust system component isa turbine housing, an exhaust manifold, or a turbine housing-integratedexhaust manifold.
 8. The ductile iron according to claim 2, wherein thespheroidization rate of graphite is 90% or higher.
 9. The ductile ironaccording to claim 3, wherein the spheroidization rate of graphite is90% or higher.
 10. The ductile iron according to claim 4, wherein thespheroidization rate of graphite is 90% or higher.
 11. An exhaust systemcomponent, produced using the ductile iron according to claim
 2. 12. Anexhaust system component, produced using the ductile iron according toclaim
 3. 13. An exhaust system component, produced using the ductileiron according to claim
 4. 14. An exhaust system component, producedusing the ductile iron according to claim 5.